III-V semiconductor materials are being used, e.g., in the manufacture of semiconductor optical devices which either emit light in response to an applied voltage or produce a voltage in response to incident light; examples of such opto-electronic devices are light-emitting diodes, super-radiant diodes, laser diodes, photodetectors, opto-isolators, and phototransistors. Particularly well developed in III-V semiconductor opto-electronics are devices consisting of a gallium arsenide or indium phosphide substrate on which a light-emitting or light-detecting p-n junction is formed in epitaxially deposited layers of suitably doped gallium arsenide, indium phosphide, or other lattice-matched III-V material. Such opto-electronic devices are typically produced in the form of tiny chips having electrodes attached to two facets which are parallel to the epitaxial layers. A survey of light-emitting diode and laser technology may be found in the paper by M. B. Panish, "Heterostructure Injection Lasers", Proceedings of the Institute of Electrical and Electronics Engineers, Vol. 64, No. 10 (October 1976), pp. 1512-1540.
For the deposition of epitaxial layers in the manufacture of III-V semiconductor devices, methods known as liquid phase epitaxy (LPE) and molecular beam epitaxy (MBE) have been used successfully, the former being considered particularly suitable for commercial production. Such production is facilitated by the use of apparatus and methods for the deposition of successive layers as described, e.g., in U.S. Pat. No. 3,565,702, issued Feb. 23, 1971 to H. Nelson and in the paper by M. B. Panish et al., "Preparation of Multilayer LPE Heterostructures with Crystalline Solid Solutions of Al.sub.x Ga.sub.1-x As: Heterostructure Lasers", Metallurgical Transactions, Vol. 2, March 1971, pp. 795-801.
Device manufacture may involve processing at elevated temperatures at which semiconductor constituent elements may have widely differing volatility; this is a particular concern in the case of epitaxial III-V materials containing indium phosphide. To aid in the realization of precisely specified composition, there is a need for an analytic tool for determining indium-to-phosphorus ratio at the surface of a III-V material.